Central Mechanism of Pain (7/10/15)

Question 1

What area of the Brainstem is associated with pain?

Answer 1

The Caudalis (Medullary dorsal horn) which is the Inferior portion of Medulla. *This is also part of the spinal trigeminal nucleus

Question 2

What Types of neurons do you find in the Medullary Dorsal Horn?

Answer 2

You have:
Nociceptive Specific Neurons
- painful mechanical stimuli 
 Wide Range Dynamic Neurons
- Light touch
- Painful mechanical stimuli
- Noxious Heat

*Note: the Nociceptors are usually more restricted to superfical layer of the MDH.

Question 3

Nociceptive specific neurons terminate mostly in ____________ layers of the MDH.

Answer 3

Superficial layers I and II

Question 4

Non-nociceptive neurons terminate mostly in _________ layers of the MDH.

Answer 4

Deeper layers III, IV and V

*There is overlap in layers II and V

Question 5

Describe Convergence in the MDH. What does it mean? and What happens?

Answer 5

Convergence refers to the phenomenon of multiple sensory receptors giving information to a smaller number of neural cells.
In the MDH Convergence leads to the emergence of wide dynamic range neurons.

Question 6

What does the Convergence of peripheral afferent with different receptive fields cause?

Answer 6

Well, when you have convergence of cutaneous, joint, muscle and other nerves it can be a substrate for referred pain.
*MDH can be substrate for referred pain.

Question 7

Is Referred pain a normal condition? What (several) factors contribute to it.

Answer 7

No! It only occurs under pathological conditions.
These factors can contribute to it:
- Convergence in MDH (Pain and non-pain) afferents converging on “pain-signaling” neuron.
- Peripheral sensitization that occurs with inflammation 7 nerve damage (However, this mostly increases c-fiber sensitivity so it cannot be solely responsible)

Question 8

What is Allodynia?

Answer 8

Feeling pain from a stimuli that should not be painful. *One of the 2 characteristics of central sensitization.

Question 9

What is Hyperalgesia?

Answer 9

Feel EXCESSIVE pain from a stimuli that shouldn’t have been that painful. *The second of the 2 characteristics of central sensitization

Question 10

What is Central sensitization?

Answer 10

Basically, when a stimuli can be either magnified or misconstrued (neurally) to be painful or sometimes both.

Question 11

What evidence do we have that supports Central sensitization?

Answer 11

If you inject capsaicin into a hand, pain will be felt at the injection and all around it.

However, if you block the A-delta and A-beta fibers pain is no localized to only where the capsaicin actually is! (Due to C-fibers)

Question 12

Describe the Process of Central sensitization…..

Answer 12

First, it begins with a barrage on C-fibers (acute pain or inflammation) this will lead to…
MDH Neuron Responses
- depolarization by substrate P (Tachykinin)
- Modification of NMDA receptors (Removal of Mg++ block)
- Increase in Conductance of NMDA receptor
Which will cause…….
Previously ineffective A-fibers to become effective!
- A-fibers release of glutamate effective at NMDA receptor now = larger receptive field.
- Response to innocuous stimuli induces pain (A-beta fibers)

Question 13

Describe Central sensitization following Pulpitis…..

Answer 13

So you have inflamed teeth on one side of your mouth and yet you still have a lower pain threshold in contralateral healthy teeth. Why? B/C the inflamed teeth are “sensitizing” central neurons with input to make Healthy teeth more sensitive!

*This is due to central sensitization and Convergence.

Question 14

Is pain confined to the MDH?

Answer 14

No..Trigeminal and pons can play a part too!

Question 15

What is a Trigeminal Tractotomy?

Answer 15

Cutting select fiber to the MDH to prevent pain.

Question 16

What are the effects of a trigeminal Tractotomy?

Answer 16

If C1-C3 (all afferents) are cut you can expect the following:

• Loss of thermal and light touch to the back of the head.
• Normal thermal and touch to face, but no pain.
• Hypolgesia in intraoral region
• Pupal pain remains intact!

Question 17

What are the effects of a central lesion in the pons?

Answer 17

Diminishes intramural and premolar pain along with intraoral touch and thermal.

Question 18

There are multiple pain pathways, what are the 3 you need to know?

Answer 18

1. Afferents -> MDH -> N. Submedius -> cingulate cortex = emotional experience of pain
2. Afferents -> MDH -> VPL (Thalamus) -> S1 (cortex) = sensory aspect of pain
3. Afferents -> MDH -> Pons and Medulla (RF and oral motor Nerve (jaw-opening reflex, increased HR and BP))

Question 19

What is the response to pain/stimuli in the VPM (Not VPL!)?

Answer 19

1. Small receptive field so it’s good at locating “Where” the stimulus is applied.
2. It tracks the onset/offset of the stimulus so it knows “when” it is applied.
3. The stimulus creates a linear response here.

Question 20

What is the repose to pain/stimuli in N. Submedius?

Answer 20

1. Larger receptive field so it’s NOT good at locating stimulus precisely.
2. The response outlasts the stimulus
3. Neural representation of negative emotion outlasts the stimulus.

Question 21

How does Phantom Pain occur?

Answer 21

Phantom pain is due to peripheral changes such as:

• sprouting
• Ectopic impulses
• Ephaptic transmission
• Up/down regulation of neurotransmitters, channels and transduction molecules

AND Central changes such as:

• Sprouting
• central sensitization (unmasking of synapses)

Question 22

The N. Submedius and Cingulate cortex do what?

Answer 22

Process the emotional component of pain

*Neural response outlasts the stimulus (Poor localization)

Question 23

The VPL and somatosensory cortex do what?

Answer 23

• Localization of pain (small receptive field)
• Tracks pain stimulus onset and offset
• May mediate phantom pain through reorganization

Question 24

How can forebrain bundles modulate pain perception?

Answer 24

• Anxiety can increase pain perception.

- Placebo effect can suppress pain perception.

Question 25

T or F, Anterior cingulate cortex is susceptible to opioids.

Answer 25

True!

Question 26

Describe the Descending control of Pain…

Answer 26

Multiple CNS sites modulate pain:

1. Forebrain ant. cingulate cortex = anxiety and placebo effect
2. Midbrain Periaquaductal grey = pain suppression
3. Rostral ventromedial medulla
   * Many of these sites contain Endogenous opioids to suppress pain!